Conventionally, in the treatment of waste containing organic products such as wood, agricultural products (e.g., sugarcane, corn), general plants (e.g., algae, grass), such products have been gasified for processing. Recently, another gasification technology has been developed for processing industrial waste having, as a major constituent, an organic substance such as plastic, recycled paper, and so-called "shredder dust." "Shredder dust" results from plastics used in automobiles that are scrapped and shredded, and from fiber-reinforced plastic (FRP) ships whose major constituent is a thermoplastic resin.
In the treatment of waste by gasification of these organic substances, a thermal decomposition furnace or combustion furnace of one-level-moving or fluid-bed type in the presence of air or oxygen is generally used. Collecting the resulting waste heat is the major process. Recently, a technology for waste containing plastics has been developed to undertake thermal decomposition and to collect oil components (hereafter referred to as the thermal decomposition method).
Also in the "fixed floor method," the two-level type described below (hereafter referred to as the air decomposition method) is employed. That is (1) in the first-level operation, the air supply is controlled within the range of about 400.degree. C. to about 800.degree. C. to decompose components for which controlled decomposition is easy; while (2) in the second-level operation, any remaining uncombusted carbon components (coke, charcoal, etc.) are combusted in the presence of oxygen at high concentrations for combustion enhancement and the like to maintain the temperature within the range of about 800.degree. C. to about 1200.degree. C.
The above-mentioned conventional methods have the following problems.
(1) Thermal Decomposition Method
(a) Plastics: Because a plurality of materials coexist in plastics, undertaking the process in the wide range of temperatures from about 300.degree. C. to about 800.degree. C. is difficult. Also, problems arise because soot is generated and coking carbon is deposited on the walls of the equipment. In addition, thermal decomposition oil is of poor quality because it contains uncombusted carbon, which limits the use of the oil to a low-quality fuel.
(b) Wood, Agricultural Products, General Plants (e.g., algae, grass), and so forth: This process produces about 40% to about 60% residual carbon. In addition, thermally combusted gases contain a large amount of carbon dioxide (CO.sub.2), so that the gases are not able to be used as fuel gases.
(c) Shredder Dust: Shredder dust has the same problems as noted above for plastics.
(2) Air Combustion Method
(a) The major components of the combusted waste gas are nitrogen (N.sub.2) and carbon dioxide (CO.sub.2), which generate a low level of heat so that the gas cannot be used as a fuel.
(b) The incomplete combustion produces soot or nitrogen oxide (NO.sub.x) as a by-product and further generates dioxins providing a secondary pollution.
(c) Plastics, particularly those which go to above 1200.degree. C. during combustion, tend to damage the walls of a combustion furnace.
To overcome these problems, the inventors previously filed a patent application (Japanese Patent Application [Tokugan] H5-290349), laid open on May 30, 1995, in which they proposed a process of gasifying waste containing organic substances which may be combusted or gasified by means of partial oxidation in the presence of air or oxygen and steam. In this process, the gasification includes the step of adjusting the molar ratio of steam/carbon (H.sub.2 O/C) for the supplied steam and the organic substances containing carbon to be between about 1 to about 10, the step of partial oxidation at about 700.degree. C. to about 900.degree. C.
According to the aforementioned Japanese Patent Application (Tokugan) H5-290349, clean or gasified gases with low soot or tar content can be obtained. However, when this process is applied to waste containing organic substances, specifically to waste having those components which are difficult to gasify, a high gasification efficiency may be difficult to obtain. This results in problems such as leaving residues having unreacted carbons as a major component and the like. The present invention resolves these problems and provides a gasification process for waste containing organic substances without discharging soot or tar from the carbon residue or from incomplete combustion outside of the system. Furthermore, the process efficiently collects heat energy.